Permanent magnet steel



Patented Mar. 7, 1944 PERMANENT MAGNET STEEL Kenneth L. Scott, WesternSprings, 111., assignor to Western Electric Company, Incorporated, NewYork, N. Y., a corporation of New York No Drawing. Application April 28,1942, Serial No. 440,875

7 Claims.

This invention relates to permanent magnet steel and a method of makingsuch steel, and more particularly to permanent magnets of such steel anda method of making the same.

Carbon manganese steel is well known as a permanent magnet material,although other steels such as chromium, cobalt, or tungsten have beenmore commonly used in recent years. However, now because of restrictionson the quantity which may be used of certain critical'materials, many ofthe ordinary magnet steels are nolonger generally available andtherefore resort must be had to combinations involving the use ofsmaller quantities of the restricted materialsv or to materials notrestricted. Thus, carbon manganese steel is again being pro-posed as apermanent magnet material.

Some difliculties have been experienced with carbon manganese steel dueto the necessity of water quenching and the consequent rapid cooling ofthe steel, which sometimes causes serious cracking and warping. It isgenerally understood that by rapid cooling through water quenching theprecipitation of carbides is held at an incipient stage and that thisproduces the desired magnetic properties. However, rapid cooling tendsto cause cracking and warping.

It is an object of the present invention to provide an improvedpermanent magnet steel and an improved method of making the same.

In accordance with one embodiment of this invention, a small percentageof chromium is added to the carbon manganese'steel melt and then thealloy is quenched in oil. An oil quench affords a substantially slowercooling rate than that afforded by a water quench, thus reducing warpingor cracking of the steel.

The above described and other objects and advantages of this inventionwill be apparent from the following detailed description:

Carbon manganese steel as used for permanent magnets heretofore had ingeneral the composition of .5% to 1% carbon. .5% to 1.25% man-' ganeseand the rest iron. While this composition produced a permanent magnetsteel having satisfactory magnetic properties, the steel had to be waterquenched to prevent rapid precipitation of carbides and consequent lossof magnetic properties. Because of the physical properties of thiscomposition, rapid cooling had the undesirable effect of causingcracking and warping of the steel. By adding .2% to .'75% chromium to amelt of carbon manganese steel of the composition just described, itappears that the former rate of precipitation of carbides issufi'iciently reduced so that slower cooling in an oil quench isfeasible. Slower cooling reduces greatly the warping and cracking of thesteel heretofore encountered. It further appearsthat an alloy comprising.5% chromium, .6% carbon, .85% manganese and the rest iron, givessatisfactory magnetic and physical properties for many purposes.

The carbon, manganese, chromiumand iron may be first melted together andthe .alloy thus produced cast into ingots and fabricated into thedesired shapes. During casting and fabrication, the alloy cools slowlyand carbides are precipitated in an excessive amount with a consequentloss of magnetic properties. The fabricated alloy is therefore reheatedsufliciently to cause the carbon to reenter solution with the iron. Withan alloy of the composition indicated above as satisfactory, atemperature of about 1450 F. appears to be slightly above the criticalpoint and sufiicient to cause the carbon to reenter solution with theiron. It will be understood that with other alloys having differentproportions, the

critical temperature may also be different. The I temperature ismaintained long. enough to permit substantially all of the carbon toreenter solution with the iron. Then the alloy is quenched in an oilquench, thus providing a rate of cooling enough lower than tha afiordedby water to pre-' vent warping and cracking,while at the time 'mium, .5%to 1% carbon, .5% to 1.25% manganes and the rest iron together to forman alloy, heating the alloy to cause the carbon to enter solution withthe iron and then cooling the alloy in an oil quench.

2. A method of making a permanent magnet comprising the steps of melting.5% chromium, .60% carbon, .85% manganese and the rest iron together toform an alloy, heating the alloy to cause the carbon to enter solutionwith the iron, and then cooling the alloy in an oil quench.

3. A method of making a permanent magnet comprising melting .2% to 375%chromium, .5% to 1% carbon, .5% to 1.25% manganese and the rest irontogether to form an alloy, fabricating a magnet, reheating the alloy tocause the carbon to enter solution with the iron, and then cooling thealloy in an oil quench.

4. A method of making a permanent magnet which comprises heating analloy of .2% to 175% chromium, .5% to 1% carbon, .5% to 1.25% manganeseand the balance iron, to cause the carbon to enter solution with theiron and then quenching the alloy in oil.

5. A method of making a permanent magnet which comprises heating analloy of .5% chromium, .60% carbon, 35% manganese and the balance iron,to cause the carbon to enter solution with the iron and then quenchingthe alloy in oil.

6. A permanent magnet comprising .2% to .75% chromium, .5% to 1% carbon,.5% to 1.25%

5 manganese and the rest iron, the alloy having 10 mium, .60% carbon,.85% manganese and the rest iron, the alloy having been heated to causethe carbon to enter solution with the iron, and cooled in an oil quench.

KENNETH L. SCO'I'I.

